Shortages of fresh water, e.g., potable water and/or water for agricultural uses are being encountered more often due to increasing demands from an increasing population, and the concentration of people in large metropolitan areas. It has been estimated that by the year 2050 some four billion people will be facing sever water shortages. Such water shortages are not limited to underdeveloped countries. It is estimated that people living in southwestern states in the United States, for example, could be facing severe freshwater shortages even earlier. Even though most of the Earth's surface is covered by water, it is estimated that less than two percent of the surface water is fresh water. Shortages of fresh water are further compounded by waste and poorly managed water supplies.
Despite the many constructive uses of fresh river water everywhere, a large amount of fresh river water flows into the world's oceans every day. Many regions, municipalities, agricultural users, and the like divert or otherwise contain large quantities of fresh river water in reservoirs which are typically located near the source of the water. However, large fresh water reservoirs are very expensive to build and maintain, and require large regions of land that might be put to other productive uses. Moreover, suitable locations for such large reservoirs are clearly limited.
A significant proportion of the population is located near the ocean or other major bodies of salt water. The salt water is generally not potable, of course, although large quantities of fresh water regularly flow into the bodies. Typically, the flow of fresh water in rivers is very seasonal, and seasonal flow forecasting is an important undertaking for most water supply systems. The seasonality of river flows is due to the seasonality of rainfall, as well as the availability of other watershed resources such as snow accumulations.
Typically, during times of high water flow fresh water is abundantly available to fill local needs, but when the water flow drops off severe fresh water shortages can occur. It would be useful to store fresh water river effluent from periods of high water flow, for use during times of low water flow.
Also, in certain regions near bodies of salt water and without an adequate fresh water source, water desalination plants are used to extract fresh water from the salt water body. In order to run the desalination plants at peak efficiency, while ensuring a stable supply of fresh water, it is desirable to have a reservoir to store fresh water that is produced, for purposes of load leveling and to accommodate periods of equipment maintenance.